Backlight with structured surfaces

ABSTRACT

A backlight includes a lightguide, a light source disposed with respect to the lightguide to introduce light into the lightguide and a turning film. Optical structures are formed in one of an output surface and a back surface of the lightguide. The optical structures are arranged to extract light from the lightguide. A back reflector is disposed adjacent the back surface. The optical structures are formed to include a varying pattern arranged to mask non-uniformities in the output of the lightguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/613,313, filed Jul. 11, 2000 which is a continuation-in-part of U.S.patent application Ser. No. 09/415,471, filed Oct. 8, 1999, now U.S.Pat. No. 6,845,212, issued Jan. 18, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a backlight and more particularly tobacklights including lightguides formed with optical structures in oneor more surfaces of the lightguide.

2. Description of the Related Technology

Backlit display devices, such as liquid crystal display (LCD) devices,commonly use a wedge-shaped lightguide. The wedge-shaped lightguidecouples light from a substantially linear source, such as a cold cathodefluorescent lamp (CCFL), to a substantially planar output. The planaroutput is then used to illuminate the LCD.

One measure of the performance of the backlit display is its uniformity.A user can easily perceive relatively small differences in brightness ofa display from one area of the display to the next. Even relativelysmall non-uniformities can be very annoying to a user of the display.

Surface diffusers or bulk diffuser sheets, which scatter the lightexiting the lightguide, are sometimes used to mask or softennon-uniformities. However, this diffusion also results in light beingdirected away from a preferred viewing axis. A net result can be areduction in overall brightness of the display along the preferredviewing axis, which is another performance measure of a display device.

From a subjective standpoint relatively small increases or decreases inoverall brightness are not as easily perceived by the user of thedisplay device as are discrete nonuniformities. However, the displaydevice designer is discouraged by even the smallest decreases in overallbrightness including decreases so small they might only be perceived byobjective measurement. This is because display brightness and powerrequirements of the display are closely related. If overall brightnesscan be increased without increasing the required power, the designer canactually allocate less power to the display device, yet still achieve anacceptable level of brightness. For battery powered portable devices,this translates to longer running times.

SUMMARY OF THE INVENTION

In accordance with the invention, an optical element, such as alightguide, optical film or lens, is formed with a predetermined,programmed pattern of optical structures. The optical structures may bearranged to selectively correct for non-uniformities in the output ofthe optical element, or may be arranged to otherwise effect theperformance of the display in a predetermined, and designed manner.

In a first aspect of the invention, an optically transmissive filmhaving a first surface and a second surface and a first edge and asecond edge is formed with a plurality of optical structures formed inthe first surface. The plurality of optical structures are arranged onthe first surface in a predetermined pattern, and each optical structurehas at least one characteristic selected from the group consisting of anamplitude, a period and an aspect ratio. Each characteristic has a firstvalue for a first predetermined location on the film between the firstedge and the second edge and the characteristic has a second value,different from the first value, for a second predetermined location onthe film, different than the first predetermined location on the film,between the first edge and the second edge.

In another aspect of the invention, the structure in accordance with theinvention is part of a thick optical element, such as for example, alightguide wedge or slab. The structure is achieved on the thick elementthrough injection molding, casting, compression molding, or by bonding afilm with the structure to the thick optical element.

BRIEF DESCRIPTION OF THE DRAWINGS

The many advantages and features of the present invention will becomeapparent to one of ordinary skill in the art from the following detaileddescription of several preferred embodiments of the invention withreference to the attached drawings wherein like reference numerals referto like elements throughout and in which:

FIG. 1 is a perspective view of an illumination device adapted inaccordance with an embodiment of the invention;

FIG. 2 is a perspective view of an optical film incorporating aprogrammed pattern of optical structures in accordance with oneembodiment of the invention;

FIG. 3 is a perspective view of an optical film incorporating aprogrammed pattern of optical structures in accordance with anotherembodiment of the invention;

FIG. 4 is a perspective view of an optical film incorporating aprogrammed pattern of optical structures in accordance with anotherembodiment of the invention;

FIG. 5 is a perspective view of a lightguide wedge incorporating aprogrammed pattern of optical structures in accordance with anotherembodiment of the invention;

FIG. 6 is a perspective view of a lightguide wedge incorporating anin-phase programmed pattern of optical structures in accordance withanother embodiment of the invention;

FIG. 7 is a cross-section view taken along line 7-7 of FIG. 6;

FIG. 8 is a perspective view of a lightguide wedge incorporating anout-of-phase programmed pattern of optical structures in accordance withanother embodiment of the invention;

FIG. 9 is perspective view of a linear lens structure incorporating aprogrammed pattern of optical structures in accordance with anotherembodiment of the invention;

FIG. 10 is a schematic plan view representation of a circular lensstructure incorporating a programmed pattern of optical structures inaccordance with another embodiment of the invention;

FIG. 11 is a schematic perspective view representation of the circularlens structure shown in FIG. 10;

FIG. 12 is a perspective view of an optical film incorporating aprogrammed pattern of optical structures in accordance with an alternatepreferred embodiment of the invention;

FIG. 13 is a perspective view of an optical film incorporating aprogrammed pattern of optical structures in accordance with an alternatepreferred embodiment of the invention;

FIG. 14 is a perspective view of an optical film incorporating aprogrammed pattern of optical structures in accordance with an alternatepreferred embodiment of the invention;

FIG. 15 is a perspective view of a lightguide incorporating a firstprogrammed pattern of optical structures in a top surface and a secondprogrammed pattern of optical structures in a bottom surface inaccordance with a preferred embodiment of the invention;

FIG. 16 is a side view illustration of the lightguide shown in FIG. 15;

FIG. 17 is an exploded perspective view of a backlight in accordancewith a preferred embodiment of the invention;

FIG. 18 is an exploded perspective view of a backlight in accordancewith a preferred embodiment of the invention;

FIG. 19 is a plot illustrating light output distribution for thebacklight illustrated in FIG. 17;

FIG. 20 is a plot illustrating light output distribution for thebacklight illustrated in FIG. 18;

FIG. 21 is a side view illustration of a backlight in accordance withthe prior art;

FIG. 22 is a side view illustration of a backlight in accordance with apreferred embodiment of the invention;

FIGS. 23-28 are side view illustrations of various configurations ofbacklights in accordance with the preferred embodiments of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in terms of several preferredembodiments, and particularly, in terms of an optical film or alightguide suitable for use in a backlighting system typically used inflat panel display devices, such as a laptop computer display or adesktop flat panel display. The invention, however, is not so limited inapplication and one of ordinary skill in the art will appreciate that ithas application to virtually any optical system, for example, toprojection screen devices and flat panel televisions. It will be furtherappreciated that the invention has application to small LCD displaydevices such as those found in cellular telephones, personal digitalassistants (PDAs), pagers, and the like. Therefore, the embodimentsdescribed herein should not be taken as limiting of the broad scope ofthe invention.

Referring to FIG. 1, an illumination system 10 includes a light source12; a light source reflector 14; a lightguide 16 with an output surface18, a back surface 20, an input surface 21 and an end surface 22; areflector 24 adjacent the back surface 20; a first light redirectingelement 26; a second light redirecting element 28; and a reflectivepolarizer 30. The lightguide 16 may be a wedge, a modification thereofor a slab. As is well known, the purpose of the lightguide is to providefor the distribution of light from the light source 12 over an area muchlarger than the light source 12, and more particularly, substantiallyover the entire area formed by the output surface 18. The lightguide 16further preferably accomplishes these tasks in a compact, thin package.

The light source 12 may be a CCFL that inputs light to the edge surface21 of the lightguide 16, and the lamp reflector 14 may be a reflectivefilm that wraps around the light source 12 forming a lamp cavity. Theback reflector 24 is located behind the lightguide 16 adjacent to theback surface 20. The back reflector 24 may be an efficient backreflector, e.g., a diffuse reflective film or a specular reflectivefilm.

In the embodiment shown, the edge-coupled light propagates from theinput surface 21 toward the end surface 22, confined by total internalreflection (TIR). The light is extracted from the lightguide 16 byfrustration of the TIR. A ray confined within the lightguide 16increases its angle of incidence relative to the plane of the top andbottom walls, due to the wedge angle, with each TIR bounce. Thus, thelight eventually refracts out of the output surface 18 and at a glancingangle thereto, because it is no longer contained by TIR. Some of thelight rays are extracted out of the back surface 20. These light raysare reflected back into and through the lightguide 16 by the backreflector 24. First light redirecting element 26 is arranged as aturning film to redirect these light rays exiting the output surface 18along a direction substantially parallel to a preferred viewingdirection.

With reference still to FIG. 1 and with brief reference also to FIG. 2,the first light redirecting element 26 may be a light transmissiveoptical film with a first surface 32 and a second surface 34. The firstsurface 32, in a turning film application, is arranged as an inputsurface and is formed with prisms 44, which refract and reflect thelight exiting the lightguide 16 along the preferred viewing direction.The second surface 34 is therefore an output surface. The prisms mayhave a substantially uniform configuration, or may have a non-uniformconfiguration as described in commonly assigned U.S. Pat. No. 6,256,391,issued Mar. 12, 2002, the disclosure of which is hereby expresslyincorporated herein by reference.

Referring back to FIG. 1, the second light redirecting element 28 maynot be required in every configuration of the illumination system 10.When included in the system 10, the second light redirecting element maybe a diffuser, a lenticular spreader or a prism film, for example abrightness enhancing film such as the 3M Brightness Enhancement filmproduct (sold as BEFII or BEFIII) available from Minnesota Mining andManufacturing Company, St. Paul, Minn. The reflective polarizer 30 maybe an inorganic, polymeric or cholesteric liquid crystal polarizer film.A suitable film is the Diffuse Reflective Polarizer film product (soldas DRPF) or the Specular Reflective Polarizer film product (sold asDBEF), both of which are available from Minnesota Mining andManufacturing Company. Furthermore, at least the second lightredirecting element 28 and the reflective polarizer 30, and potentiallythe first light redirecting element 26, may be combined into a singleoptical element. The commonly assigned US Patent Application entitled“DISPLAY ILLUMINATION DEVICE AND METHOD OF ENHANCING BRIGHTNESS IN ADISPLAY ILLUMINATION DEVICE,” Ser. No. 09/415,100, filed Oct. 8, 1999,the disclosure of which is hereby expressly incorporated herein byreference, describes several such combined optical structures.

With lightguides used for backlighting, such as the lightguide 16, it iscommon for there to be non-uniformities in the light output from thelightguide. These non-uniformities can frequently be concentrated nearthe input surface 21. To mask non-uniformities, which are generallyconsidered a defect, a diffuser that covers the output surface of thelightguide is typically used. However, a diffuser tends to reduce theoverall brightness of the display and may not adequately mask all of thedefects.

As described above, in the illumination system 10, the first lightredirecting element 26 is arranged as a turning film, and may have astructure as shown in FIG. 2. Referring once again to FIG. 2, the filmcontains a pattern 42 of optical structures 40 (prisms) that arearranged to have an out-of-phase varying amplitude. For a turning filmapplication, the pattern 42 is formed on a surface that is the lightinput surface of the film. However, in other applications several ofwhich will be described herein, the pattern 42 may be formed on a topand/or bottom surface of a wedge, slab or film. For the turning filmapplication illustrated in FIG. 1, in addition to the prisms formed onthe first surface 32 of the first light redirecting element 26, thesecond surface 34 may be formed with optical structures.

Continuing with the discussion in connection with FIG. 2, the firstlight redirecting element 26 has a first edge 36 and a second edge 38.The optical structures 40 extend from the first edge 36 toward thesecond edge 38 in the pattern 42. Each optical structure 40 may have anumber of characteristics, such as amplitude, period and aspect ratio ofthe peaks 44 and valleys 46. The pattern 42 may also havecharacteristics, such as for example, a pitch, p, between opticalstructures 40. The structures 40 in FIG. 2 are shown having amplitudevariation. In application of the first light redirecting structure 26,the grooves may be arranged such that variation in their amplitude isperpendicular to the lightsource 12 (FIG. 1).

With continued reference to FIG. 2, it is observed that within thepattern 42, the optical structures 40 are formed with a larger amount ofamplitude variation at the first edge 36, and this amplitude variationdecreases in magnitude toward the second edge 38. The larger amount ofamplitude variation in the optical structures 40 produces more opticalpower along the groove axis because of the higher surface slopes. Theoptical power of this pattern then decreases as a function of thedistance from the first edge 36. This arrangement of the opticalstructures 40 and the pattern 42 is purposeful. As noted,non-uniformities in the output of lightguide 16 may be concentrated nearthe input surface 21 while there may be less non-uniformity farther fromthe input surface 21. Thus, the optical structures 40 and the pattern 42are arranged to provide more diffusion near the first edge 36. Inapplication, the first edge 36 will be disposed substantially adjacentthe input surface 21 of the lightguide 16. The pattern 42 has a pitch,p, which may be uniform or variable, and the amplitude of the opticalstructures 40 may decrease to naught toward the second edge 38. Thispattern, as will be discussed in more detail below, may be produced withany tool shape.

It should be appreciated that using ray tracing and other analysistechniques, it is possible to determine particular arrangements for theoptical structures 40 and the pattern 42 that best correct particularobserved non-uniformities in the output of the lightguide 16. That is,one or more of the characteristics of the optical structures 40 and thepattern 42 may be tailored to correct a particular non-uniformity. Asdescribed above, in connection with first light redirecting element 26,the optical structures 40 and the pattern 42 provided optical power tothe output of the lightguide 16 near the input surface 21 in order tomask non-uniformities that may occur near the input surface 21. Less orno optical power is provided away from the input surface 21 as fewer orless intense non-uniformities are typically observed from the lightguide16 farther from the input surface 21. In this manner, optical power isprovided where most needed to mask or soften non-uniformities, whileless optical power is provided where there may be less non-uniformity tomask. Moreover, optical power may be added virtually anywhere to theoutput of the lightguide by adding optical structures and/or varying thecharacteristics of the optical structures. Furthermore, the addition ofoptical power need not be uniform. Instead, optical power may be added,as necessary, to discrete regions of the lightguide output if necessaryto help mask a defect or create a particular optical effect.

Some lightguides include a pattern of diffuse dots on a back surface ofthe lightguide. Light incident to one of the dots is diffusely scatteredby the diffuse dot, and a portion of this reflected light is caused toexit the light guide. In spite of the diffuse nature of this method ofextracting light from the lightguide, the pattern of dots may itself bevisible in the lightguide output. Thus, to hide the dot pattern,additional diffusion is typically provided.

With reference to FIG. 3, a film 50 has a surface 52 which is formed toinclude a plurality of optical structures 54 disposed in a pattern 56.The optical structures 54 are arranged essentially to replace thediffuse dot pattern for providing extraction of light from thelightguide. While shown in FIG. 3 as ellipses, the optical structures 54are not collectively limited to any particular shape nor are theylimited to any one particular shape within the pattern 56. Therefore,the optical structures 54 may be prisms, lines, dots, squares, ellipses,circles, diamonds or generally any shape or combinations of shapes.Moreover, the optical structures 54 may be made very small in size andmay be spaced very closely together within the pattern 56, much more sothan the dots within a diffuse dot pattern may be size and spaced. Forexample, the optical structures may have a size up to the size typicalof that used for diffuse dots, but preferably will be smaller than theacuity of the human eye, and may be spaced within about 50-100 μm ofeach other. This very small size and close spacing of the opticalstructures 54 eliminates or reduces the need for diffusion in the outputof the lightguide that is ordinarily necessary to hide the pattern ofdiffuse dots.

Referring to FIG. 4, an optical film 51 has a surface 53 which is formedwith a plurality of optical structures 55 disposed in a pattern 57. Inthis embodiment of the invention, the optical structures 55 are formedas circles or dots. FIG. 5 illustrates a lightguide wedge 59 with a backsurface 61 that is formed with optical structures 63 disposed in apattern 65. The optical structures again are illustrated as circles ordots, but it will be appreciated that the optical structures may take onvirtually any configuration.

The invention permits and provides for the changing of the slope of thelightguide at a micro-level. That is, the slope of the lightguide may belocally increased or decreased by the addition of optical structures atthe micro-level. When a light ray hits a higher positive slope, it willbe extracted from the lightguide faster than if it hit the nominal wedgeangle.

While so far discussed in terms of optical films, the invention hasapplication to the lightguide wedge itself. Referring to FIGS. 6 and 7,a lightguide 60 has an input surface 62, an output surface 64 and a backsurface 66. The input surface 62 is arranged to be disposed adjacent alight source (not depicted) to provide a source of light incident to theinput surface 62. The light incident to the input surface 62 isextracted out of the output surface 64 as a result of frustrated TIRwithin the lightguide 60. As discussed above, it is common for there tobe non-uniformities in the light output from the lightguide 60,particularly near the input surface 62.

FIG. 7 illustrates the addition of optical power to the back surface 66of the lightguide 60 and the adjustment in intensity extending away fromthe input surface 62. As shown in FIG. 6, the back surface 66 is formedwith in-phase optical structures 68 arranged to enhance extraction nearthe input surface 62 and to taper to naught away from the input surface62. The pattern can also be non-tapering, i.e., constant, over theentire surface, increasing from naught, randomly varying, or distributedin discrete regions. It is also possible for the optical structures tobe out-of-phase, such as optical structures 68′ formed in a back surface66′ of the lightguide 60′ shown in FIG. 8. It will be appreciated thatpatterns of optical structures may also be formed in the output surface64 either separately or in conjunction with a pattern formed in the backsurface 66—such embodiments of the inventions being described more fullybelow and particularly in connection with FIGS. 15 and 16. Returning tothe present discussion, a purpose of providing the optical structures isto achieve an effect that minimizes non-uniformities of the lightguideoutput wherever they may occur. For example, the lightguide 60 shown inFIGS. 6 and 8 may have non-uniformities that appear primarily adjacentthe input surface 62, which would suggest adding optical structures thathave more optical power near the input surface 62.

With particular reference to FIG. 7, the optical structures 68 may beformed on a surface 72 of an optical film 70. The optical film 70 maythen be coupled to the wedge structure of the lightguide 60 usingultraviolet (UV) curing, pressure sensitive or any other suitableadhesive. Alternatively, the wedge may be molded in bulk to include theoptical structures 68 in the back surface 66.

As will be more generally appreciated from the foregoing discussion,virtually any configuration of optical structures may be formed into anoptical film, and the optical film coupled, for example by bonding, to alightguide or other bulk optical element. For example, glare reduction,anti-wetout, Fresnels, and virtually any other structure that may beformed in a surface of an optical film may be easily replicated into thefilm and then the film coupled to another optical element.

Films incorporating programmed optical structures may be manufacturedusing a microreplication process. In such a manufacturing process, amaster is made, for example by cutting the pattern into a metal roll,and the master is used to produce films by extrusion, cast-and-cure,embossing and other suitable processes. Alternatively, the films may bemanufactured by compression or injection molding, casting or rollforming. A preferred apparatus and method for microreplication isdescribed in the commonly assigned U.S. Pat. No. 6,322,236 issued Nov.27, 2001, the disclosure of which is hereby expressly incorporatedherein by reference.

As an example of the above-described feature of the invention, and withreference to FIG. 9, a linear Fresnel lens or prism 80 has asubstantially planar first surface 82 and a second surface 84. Thesecond surface 84 is formed with lens structures 86 and superimposed onthe lens structures 86 are additional optical structures 88. The opticalstructures 88 have characteristics, such as amplitude, period, andaspect ratio, which vary from a first edge 90 of the lens 80 to a secondedge 92 of the lens 80. The lens 80 may be formed in bulk, or as shownin FIG. 9, the lens structures 86 including the optical structures 88may be formed on a film 94 that is then bonded to a bulk opticalsubstrate 96. Depending on the application, the first surface 82 may bearranged as an input surface and the second surface 84 as an outputsurface, and vice-versa.

FIGS. 10 and 11 illustrate schematically a circular lens 81 thatincludes a first surface 83 and a second surface 85. The second surface85 is formed to include lens structures 87, for example, circularFresnel lens structures, and superimposed over the lens structures 87are additional optical structures 89. The optical structures 89 havecharacteristics, such as amplitude, period, and aspect ratio, which mayvary, for example, from an outer circumference of the lens 81 to thecenter of the lens 81.

Referring now to FIG. 12, shown graphically is a film 100 containing avarying amplitude pattern 102 of optical structures 108 formed using a“V” shaped cutting tool. The pattern 102 may be formed on a top and/orbottom surface of the film 100. Likewise, the pattern 102 may be formedin a wedge or slab. The film 100 has a first edge 104 and a second edge106. The optical structures 108 extend from the first edge 104 towardthe second edge 106 arranged in the pattern 102. Each optical structure108 may have a number of characteristics, such as amplitude, period andaspect ratio. The pattern 102 may also have characteristics, such as forexample, a pitch, p, defining a spacing between optical structures 108.The optical structures 108 in FIG. 12 are shown having amplitudevariation. In application of the film 100, the grooves may be arrangedsuch that the variation in amplitude is perpendicular, parallel or at anangle to a lightsource of the lightguide incorporating the film 100.

With continued reference to FIG. 12, it is observed that within thepattern 102, the optical structures 108 are formed with larger amplitudeat the first edge 104 and decrease in amplitude toward the second edge106. The larger amplitude produces more optical power along the grooveaxis because of the higher surface slopes. The optical power of thispattern then decreases as a function of the distance from the first edge104. This arrangement of the optical structures 108 and the pattern 102is purposeful.

With reference to FIGS. 13 and 14, films 110 and 112, are shownrespectively. Each film 110 and 112 has characteristics like film 100,and like reference numerals are used to describe like elementstherebetween. As opposed to the pattern created by using a “V” shapedtool, the film 110, FIG. 13, has a pattern 114 of optical structure 116that is formed using a tool having a curve or arc configuration. Thefilm 112, FIG. 14, has a pattern 118 of optical structures 120 that isformed using a flat nose tool. The patterns 114 and 118 are arranged asdescribed to provide optical power in the surface or surfaces of thefilms 110 and 112. It will be appreciated that virtually any toolconfiguration may be used with the particular tool being selected toachieve a desired amount and form of optical power in the surface orsurfaces of the film.

In the lightguide 121 illustrated in FIGS. 15 and 16, a first pattern122 of optical structures 124 is formed in a bottom surface 126 and asecond pattern 128 of optical structures 130 is formed in a top surface132 of the wedge 134. For purposes of illustration only, the opticalstructures 124 are shown in FIG. 15 to extend only partially across thebottom surface 126, and the optical structures 130 are shown in FIG. 15to extend only partially across the top surface 132. It will beappreciated that the optical structures 124 and the optical structures130 will in most cases extend across the entirety of the bottom surface126 and the top surface 132, respectively. The first pattern 122 may bearranged to facilitate the extraction of light from the wedge 134, whilethe second pattern 128 may be arranged to mask non-uniformities in thelight output from the wedge. It will be appreciated, however, that thepatterns implemented in the wedge 134 will depend on the desired lightoutput to be achieved from the wedge 134. Moreover, as described above,the patterns 122 and 128 may be formed first in optical films that arelater coupled to the wedge, for example, by bonding. In anotherconstruction, surfaces 122 and 128 are formed in the wedge by injectionmolding or casting.

As is appreciated from the foregoing discussion, and in accordance withthe preferred embodiments of the invention, a lightguide may be formedwith optical structures, e.g., “V” grooves, in either a first surface, asecond surface or both. Whether the first surface or the second surfaceis an input surface relates to the orientation of the surface withrespect to a light source. The optical structures may be uniformly orrandomly spaced, and may have various other characteristics. Thus, theinvention has application to lightguides and backlight systems for avariety of applications. One example of an application is a backlightsystem that extracts light by the frustration of total internalreflection where the lightguide is formed with optical structures ineither a back surface and/or an output surface thereof. Still anotherexample is a backlight system that has a lightguide that uses a patternof dots to extract light ad includes optical structures formed in eitheror both of its back and output surfaces. These and other examples aredescribed in more detail below.

Referring to FIG. 17, a backlight 140 is illustrated and includes alight source 142 adjacent an input edge 143 of a wedge lightguide 144. Aback reflector 146 is disposed adjacent a back surface 154 of thelightguide 144, and a turning film 148 is disposed adjacent an outputsurface 150 of the lightguide 144. The back surface 154 is formed withoptical structures 152. The optical structures 152 may be grooves formedin the back surface 154, and are shown as such in FIG. 16. The groovesshown in FIG. 17 are “V” grooves and have a prism angle of about 90degrees, but prism angles ranging from 60 degrees-120 degrees may beused. Shapes other than “V” grooves may also be used for opticalstructures 152. Furthermore, each optical structure may be formed tohave a height that varies along its length from a nominal value. Thisvariation may have a wavelength, which may be in the range of about 1μm-1000 μm, preferably be less than about 140 μm. Such structures aredisclosed and described in the commonly assigned United States PatentApplication entitled “Optical Film,” Ser. No. 09/025,183, filed Feb. 18,1998 (attorney docket no. 53772USA6A), the disclosure of which is herebyexpressly incorporated herein by reference.

The optical structures 152 are shown oriented substantiallyperpendicular to the light source 142. It will be appreciated that theoptical structures 152 may be oriented parallel to the light source 142or at an angle between 0 degrees-90 degrees to the light source 142.

The turning film 148 may be any suitable prismatic turning film. Forexample, the turning film 148 may be formed as described in theaforementioned United States Patent Application entitled “Optical FilmWith Variable Angle Prisms.”

The back surface 154 is formed to include the optical structures 152.This results in some additional light being extracted from thelightguide 144 through the output surface 150 as compared to the lightthat is extracted from the back surface 154. A portion of the lightexiting the back surface 154 will encounter the back reflector 146 andwill be reflected back through the lightguide 144 and the output surface150.

Referring now to FIG. 18, a backlight 140′ is illustrated that issimilar in construction to the backlight 140, and like referencenumerals are used to designate like elements. Primed reference numeralsare used to designate elements that are altered from the backlightconstruction shown in FIG. 17. The backlight 140′ includes a lightsource 142 adjacent an input edge 143 of a wedge lightguide 144′. A backreflector 146′ is disposed adjacent a back surface 154′ of thelightguide 144′, and a turning film 148 is disposed adjacent an outputsurface 150′ of the lightguide 144′. The output surface 150′ is formedwith optical structures 152′. The optical structures 152′ may be groovesformed in the output surface 150′, and are shown as such in FIG. 17. Thegrooves shown in FIG. 18 are “V” grooves and have a prism angle of about90 degrees, but prism angles ranging from 60 degrees-120 degrees may beused. Shapes other than “V” grooves may also be used for opticalstructures 152′. Furthermore, each optical structure 152′ may be formedto have a height that varies along its length from a nominal value. Thisvariation in height may have a wavelength, which may be in the range ofabout 1 μm-1000 μm, but for lightguide applications will preferably beless than about 140 μm. Such structures are disclosed and described inthe aforementioned United States Patent Application entitled “OpticalFilm,” Ser. No. 09/025,183.

The optical structures 152′ are shown oriented substantiallyperpendicular to the light source 142′. It will be appreciated that theoptical structures 152′ may be oriented parallel to the light source142′ or at an angle between 0 degrees-90 degrees to the light source142.

Forming the output surface 150′ to include the optical structures 152′results in additional light being extracted from the lightguide 144through the back surface 154′ as compared to the output surface 150′.Some light is also extracted from the output surface 150′. The portionof the light exiting the back surface 154′ will encounter the backreflector 146′ and will be reflected back through the lightguide 144′and the output surface 150. Therefore, with the backlight 140′, it maybe desirable to directly secure the back reflector 146′ to the backsurface 154′. This may be accomplished by laminating the back reflector146′ to the back surface 154′. Such an arrangement for the backreflector 146′ is disclosed and described in the commonly assigned U.S.Pat. No. 6,447,135 issued Sep. 10, 2002, the disclosure of which isexpressly incorporated herein by reference. Alternatively, the backreflector may be formed on the back surface using a vapor depositionprocess. In embodiments in which the reflector is directly secured tothe back surface of the lightguide, it will be appreciated that thereflector should be both specular and highly efficient with very lowabsorption.

As described above, variation is added to a characteristic of theoptical structures 152 and 152′ formed respectively in the back surfaceor the output surface of the lightguide, e.g., variation in theamplitude of the optical structures, to reduce non-uniformities in theoutput of the backlight 140 and 140′, respectively. It is possible toprovide similar variation in the optical structures by other methods,such as by bead blasting the optical structures, however forming thegrooves with the described variation in prism height provides acontrollable, predictable and hence preferred method of reducingnon-uniformities in the output of the backlight.

FIG. 19 illustrates light output in a viewing cone disposed above anoutput of the backlight 140, i.e., the light exiting the backlight 140from an output surface of the turning film 148. What may be determinedfrom the illustrated light output is the on-axis luminance, the maximumluminance, the integrated intensity, the horizontal distribution orhorizontal half-angle and the vertical distribution or verticalhalf-angle. FIG. 20 provides a similar distribution for the backlight140′. Clearly noticeable is that the output of backlight 140′ has areduced horizontal distribution and a slightly increased verticaldistribution. Overall integrated intensity, or the total amount outputlight from the backlight 140 and 140′ is about the same, althoughon-axis luminance and maximum luminance is substantially increased forthe backlight 140′ as compared to the backlight 140. Appreciated fromthe FIGS. 19 and 20, is that the arrangement of optical structures inthe lightguides 140 and 140′, respectively, will have an effect on theoutput of the backlight system. In the backlight 140′, the lightguide144′ with optical structures 152′ formed in its top surface, additionalcollimation of the light output of the backlight 140′ is achieved ascompared to the backlight 140. Furthermore, because optical structures152′ may be formed with varying characteristics, as described above, thelight output from the backlight 140′ may be made uniform withoutadditional optical films or other devices, such as diffusers.

There are additional advantages associated with providing the opticalstructures 152′, including varying characteristics, in the outputsurface 150′ of the lightguide 140′. One such advantage relates to theinterface of the output surface 150′ with the turning film 148. With theoptical structures 152′ being formed in the output surface 150′, therewill be relatively few points of contact between the prisms of theturning film 148 and the output surface 150′. This may result in adecrease in the optical defect generally referred to as wet-out. Asmentioned above, providing variation in the formation of the opticalstructures 152′ helps also to mask defects in the output of thebacklight making the light output more uniform. Therefore anotheradvantage of providing the optical structures 152′ in the output surface150′ may be the elimination of a diffuser film in the overall backlightsystem. Because the optical structures 152′ provide light collimation,as may be observed from FIG. 20, it is possible, in accordance with theinvention, to provide a backlight system that requires fewer sheets ofoptical film as compared to typical backlight systems.

Illustrated in FIG. 21, are a lightguide 151, a turning film 153, an LCDdisplay 154 and a back reflector 155. Light is extracted from thelightguide 151 from both the top surface 161 and the back surface 157.It is possible that strong Fresnel reflections 156 between the backreflector 155 and the back surface 157 may trap a substantial portion ofthe light extracted from the back surface 157. This light is ultimatelylost leading to inefficiency. To improve this situation, illustrated inFIG. 22, the reflecting surface 158 of the back reflector 155′ may beformed with optical structures 159. The optical structures 159 may befacets, grooves or other shaped structures. The optical structures 159help to reduce the specular component of reflection from back reflector155′ and to direct more light up through the lightguide 151, thusincreasing its efficiency. A suitable back reflector including opticalstructures is the enhanced diffuse reflector (EDR) film product sold by3M. One of skill in the art will appreciate that the principle taught inFIG. 22 may be applied to virtually any backlight, including withoutlimitation backlight 140 and backlight systems in accordance with theadditional preferred embodiments herein described.

Several adaptations, enhancements and modifications of the backlightsystems have been described above. Still others can be appreciated andare within the scope of the invention. It will be appreciated that theparticular arrangement of the backlight system will depend on theapplication for which it is intended. To illustrate the adaptability ofthe present invention, several examples are shown and described inconnection with FIGS. 23-28.

Grooves in the Back Surface of the Lightguide

In FIG. 23, a backlight 160 includes a light source 162, a wedgelightguide 164, a back reflector 166, a turning film 168 and an optionaladditional optical film 170. The lightguide 164 has an output surface165 and a back surface 172 that is formed with optical structuressimilar to optical structures 152 shown in connection with thelightguide 144 in FIG. 16. The optical structures may be formed directlyinto the lightguide 164 by injection molding or casting. Alternatively,the optical structures may be formed in a light transmissive film thatis laminated to the back surface 172 of the lightguide 164.

With optical structures formed on the back surface 172 of the lightguide164 additional light exits the lightguide 164 through the output surface165 as compared to the back surface 172. The light exiting the backsurface 172, however, encounters the back reflector 166, and isreflected back through the lightguide 164. A suitable reflectorincluding optical structures is a grooved diffuse reflector.

In accordance with additional aspects of the backlight 160, the turningfilm 168 may be formed to include a diffusive structure in its outputsurface 176. The optional optical film 170 may be a brightness enhancingfilm, such as aforementioned BEFIII optical film, the Diffuse ReflectivePolarizer film product (sold as DRPF) or the Specular ReflectivePolarizer film product (sold as DBEF), all of which are available fromMinnesota Mining and Manufacturing Company.

Grooves in the Output Surface of the Lightguide

In FIG. 24, a backlight 180 includes a light source 182, a wedgelightguide 184, a back reflector 186, a turning film 188 and an optionaloptical film 190. The lightguide 184 has an output surface 192 that isformed with optical structures similar to optical structures 152′ shownin connection with the lightguide 144′ illustrated in FIG. 17. Thelightguide 184 may be formed by injection molding or casting so as toinclude the optical structures in the output surface 192. Alternatively,the optical structures may be formed in a light transmissive film thatis laminated to the output surface 192 of the lightguide 184. Such anarrangement potentially increases manufacturing flexibility and reducesmanufacturing costs by simplifying mold design for the lightguide 184.Instead of having a unique mold for each lightguide, lightguides may beadapted in accordance with the invention by laminating a surface of thelightguide with the optical film formed with the optical structures.

With optical structures formed on the output surface 192 of thelightguide 184 an additional amount of light exits the lightguide 184from the output surface 192 as compared to the amount of light exitingthe lightguide from a back surface 193. The light exiting the backsurface 193, however, encounters the surface 194 of the back reflector186, and is reflected back through the lightguide 184. To ensure a highpercentage of the light exiting the back surface 193 is reflected backthrough the lightguide 184, the back reflector 186 is preferablydirectly secured to the back surface 193. This may be accomplished bylaminating a mirror or mirror film to the back surface 193 or by vapordeposition coating the back surface 193. When directly secured to theback surface 193, the back reflector should be specular and highlyefficient.

In accordance with additional aspects of the backlight 180, the turningfilm 188 may be formed to include a diffusive structure in its outputsurface 196. The optical film 190 may be a brightness enhancing film,such as aforementioned BEFIII optical film, the Diffuse ReflectivePolarizer film product (sold as DRPF) or the Specular ReflectivePolarizer film product (sold as DBEF), all of which are available fromMinnesota Mining and Manufacturing Company.

In FIG. 25, a backlight 220 includes a light source 222, a wedgelightguide 224, a back reflector 226 having a surface 234 and a turningfilm 228 having a surface 236. The lightguide 224 has an output surface230 that is formed with optical structures (not depicted). The opticalstructures may have a varying pattern, such as described in theaforementioned United States Patent Application entitled “Optical Film,”formed using a cutting tool of any suitable shape. The opticalstructures may be formed directly in the lightguide 224 by injectionmolding or casting, or alternatively, the optical structures may beformed in a light transmissive film that is laminated to the outputsurface 230 of the lightguide 224.

With optical structures formed on the output surface 230 of thelightguide 224 an additional amount of light exits the lightguide 224through the back surface 232 as compared to the amount of light thatexits through the output surface 230. This light encounters the surface234 of the back reflector 226, and is reflected back through thelightguide 224. A suitable reflector may be a grooved diffuse reflector.The optical structures may also provide for masking of non-uniformities,and thus eliminate the need for a diffuser in the backlight system.

Also, because the optical structures may also provide collimation of thelight exiting the lightguide (see FIG. 20), it is possible, inaccordance with the invention, to provide a backlight system thatrequires fewer sheets of optical film as compared to typical backlightsystems. In the embodiment shown in FIG. 25 there is a single, optional,optical film 238, which may be the Diffuse Reflective Polarizer filmproduct (sold as DRPF) or the Specular Reflective Polarizer film product(sold as DBEF) available from Minnesota Mining and ManufacturingCompany.

Recycling Backlight Systems

In FIG. 26, a backlight 200 includes a light source 202, a wedgelightguide 204, a back reflector 206 having a surface 216, a turningfilm 208 having a surface 218 and one or more additional, optionaloptical films 210 and 212. The lightguide 204 has a front surface 215and a back surface 214 that is formed with optical structures similar tooptical structures 152 shown in connection with the lightguide 144 inFIG. 17. The optical structures may be formed directly in the lightguide204 by injection molding or casting. Alternatively, the opticalstructures may be formed in a light transmissive film that is laminatedto the back surface 214 of the lightguide 204.

The optical structures formed on the back surface 214 of the lightguide204 facilitate the extraction of light from the lightguide 204. Theoptical structures may therefore allow for the elimination of thediffuse dot pattern typically used to extract light from the lightguide.Some light exits the back surface 214, and this light encounters theback reflector 206, and is reflected back through the lightguide 204. Asuitable back reflector is the enhanced diffuse reflector (EDR) filmproduct sold by 3M.

Elimination of the dot pattern for extraction of light from thelightguide 204 may reduce the need to add diffusion to mask theappearance of the dot pattern in the output of the backlight 200. Theoptional optical films 210 and 212 may be brightness enhancing films,such as the aforementioned BEFIII optical film product arranged in acrossed arrangement; Diffuse Reflective Polarizer film product (sold asDRPF) the Specular Reflective Polarizer film product (sold as DBEF)and/or various combinations thereof and all of which are available fromMinnesota Mining and Manufacturing Company.

In FIG. 27, a backlight 240 includes a light source 242, a wedgelightguide 244, a back reflector 246, a diffuser 248 and first andsecond optional additional optical films 250 and 252. The back reflector246 is preferably secured to a back surface 254 of the lightguide 214using a dot patterned adhesive, such as described in the aforementionedUnited States Patent Application entitled “Lightguide Having a DirectlySecured Reflector.” The adhesive is therefore arranged in a dot patterntypical of an extraction dot pattern.

The lightguide 244 has an output surface 255 that is formed with opticalstructures (not depicted). The optical structures may have a varyingpattern as described above. The optical structures may be formeddirectly in the lightguide 244 by injection molding or casting, oralternatively, the optical structures may be formed in a lighttransmissive film that is laminated to the output surface 255 of thelightguide 244.

The optical structures including the varying pattern, as described, mayeliminate the need for a diffuser, such as the diffuser 248, to mask thedot pattern, as well as other non-uniformities in the output of thebacklight 240. As such, the diffuser 248 is optional. When used, theoptional optical films 250 and 252 may be brightness enhancing films,such as the aforementioned BEFIII optical film product, arranged in acrossed arrangement, the Diffuse Reflective Polarizer film product (soldas DRPF) or the Specular Reflective Polarizer film product (sold asDBEF), all of which are available from Minnesota Mining andManufacturing Company.

Pseudo-Wedge Backlight System

Referring now to FIG. 28, a backlight 260 includes a light source 262and a pseudo-wedge lightguide 264. The pseudo-wedge lightguide 264includes a first surface 266 and a second surface 268. The first surfacemay be formed with optical structures 270, such as optical structures152 described in connection with FIG. 17. The second surface is formedwith faceted groove structures 272 that are arranged to be parallel tothe light source 262. The faceted groove structures 272 facilitateextraction of light from the lightguide by enhancing the frustration oftotal internal reflection. Not shown, the backlight 260 will alsoinclude a back reflector disposed adjacent the second surface 268.

The faceted groove structures 272 may have variable angle features. Eachindividual facet has a facet angle. When the faceted groove structures272 include a variable angle feature, the individual facet angles varyfrom facet to facet. This arrangement of the faceted groove structures272 may reduce the appearance of nonuniformities in an output of thebacklight 260.

While the lightguide 264 is shown as a slab structure, the lightguide264 may be wedge. Furthermore, the faceted groove structures 272 may beformed directly in the lightguide 264, for example by molding orcasting, or the faceted groove structures may be formed in an opticalfilm that is laminated to a slab or wedge lightguide. The faceted groovestructures may also vary in density as a function of distance from thelight source 262.

Still other modifications and alternative embodiments of the inventionwill be apparent to those skilled in the art in view of the foregoingdescription. This description is to be construed as illustrative only,and is for the purpose of teaching those skilled in the art the bestmode of carrying out the invention. The details of the structure andmethod may be varied substantially without departing from the spirit ofthe invention, and the exclusive use of all modifications which comewithin the scope of the appended claims is reserved.

1. A lightguide comprising: an optically transmissive substrate having afirst surface, an opposite second surface, and an edge surface; and anoptically transmissive film bonded to the first surface of the substrateand having a plurality of optical structures formed therein, the lightguide being adapted to receive light from a light source through theedge surface of the substrate, the plurality of optical structuresextracting the received light out of the lightguide.
 2. The lightguideof claim 1, wherein the substrate is a wedge.
 3. The lightguide of claim1, wherein the substrate is a slab.
 4. The lightguide of claim 1,wherein the optical structures are spaced within about 50 to 100 micronsof each other.
 5. The lightguide of claim 1, wherein the opticalstructures are arranged in a pattern to correct non-uniformities inlight exiting the first surface of the substrate.